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May, 05/27/2015
Events and times subject to change

May 27, 2015 Wednesday 3:00 PM  +
Meyer 6th Floor Conference Room
Hard Condensed Matter Seminars (hcmp)

Ignace Jarrige
Brookhaven National Laboratory

Cooking up New Energy Materials Using NSLS-II: Selected Recipes for Resonant Inelastic X-ray Scattering

Electronic structure is the trait d’union between atomic layout, basic physical properties and device-scale functionality in material systems. Advances in relevant experimental probes are therefore crucial to the development of future technologies. Resonant inelastic X-ray scattering (RIXS) combines the potential of X-ray emission and X-ray absorption spectroscopies, simultaneously probing the occupied and unoccupied valence states in an element and orbital-specific fashion. In this talk I will aim at capturing some of the current excitement in the field of RIXS by reporting on recent selected studies of energy materials such as automotive catalysts, iron-based superconductors, and rare-earth systems with a tunable Kondo effect. These studies will describe how RIXS can be used to delve in the details of the electronic structure pertaining to a material’s performance, and offer paths to optimize specific applications.
I will also touch on the future of RIXS with a synopsis of the capabilities and status of the Soft Inelastic X-ray scattering (SIX) beamline, currently under construction at the recently commissioned light source NSLS-II. The beamline aims at achieving a world-class energy resolving power of 100,000, expected to lead to breakthroughs in basic and materials science.

May 28, 2015 Thursday 11:00 AM  +
Meyer 6th Floor CSMR Area
Hard Condensed Matter Seminars (hcmp)

Javad Shabani

Two Dimensional Epitaxial InAs-Al Superconducting Systems

It has been recently realized that materials with strong spin orbit coupling can lead to novel states of matter such as topological insulators and superconductors. This exciting development might lead to a number of useful applications ranging from spintronics to quantum computing. Here, we present epitaxial growth of near surface InAs heterostructures where the structure is designed such that the charge distribution is mainly confined in InAs but has a non-zero value at the surface. This band engineering allows us to make ohmic contact to the 2DES while keeping the electron mobilities well above traditional surface structures. In addition, the Rashba parameter measured from weak antilocalization analysis exhibit a very large spin-orbit coupling due to highly asymmetric potentials near surface. We also show that near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on near-surface InAs 2DESs. Indeed, superconductor-semiconductor-superconductor junctions fabricated on these wafers exhibit a supercurrent with superior properties compared to ex-situ deposition of superconducting metals. The supercurrent flowing through the semiconductor can be controlled by a gate voltage making a transistor where the current can be turned “off” and “on”. Our results show that these new systems could provide new possibilities to study new quantum transport phenomena in addition to topological superconductivity.

May 28, 2015 Thursday 2:00 PM  +
Meyer 6th Floor Confrence Room
Hard Condensed Matter Seminars (hcmp)

Alejandrao Cabo
Instituto de Cibernética, Matemática y Física

Tight Binding Model and Phenomenology of the High Temperature Superconductors

Former results for a Tight-Binding (TB) model of CuO planes in La2CuO4 are reviewed here to underline their wider implications. It is noted that physical systems being appropriately described by the TB model can exhibit the main strongly correlated electron system (SCES) properties, when they are solved in the HF approximation, by also allowing crystal symmetry breaking effects and non-collinear spin orientations of the HF orbitals. In particular, it is argued how a simple 2D square lattice system of Coulomb interacting electrons can exhibit insulator gaps and pseudogap states, and quantum phase transitions as illustrated by the mentioned former works. These results allow to understand the nature of the observed quantum phase transition laying “beneath” the superconducting Dome. It corresponds to coalescence under hole doping increase, of an insulator ground state (with a highly degenerated spin order) with an excited pseudogap state, showing a lattice order symmetry breaking. The evolution of the band structure and Fermi surface with doping is determined. It shows a drastic change at a critical hole density xc = 0.2, which are in close coincidence with the experimental observations. Above this critical density the system becomes a paramagnetic metal. A discussion will be also presented indicating the possibility of attaining room temperature superconductivity, by means of a surface coating with water molecules, of cleaved planes of graphite, being orthogonal to its c-axis. The possibility that 2D arrays of quantum dots can give rise to the same effect is also proposed to consideration. The analysis also furnishes theoretical insights to solve the Mott-Slater debate, at least for the La2CuO4 and TMO band structures.

September 18, 2015 Friday 11:00 AM  +
Meyer 5th Fl. CCPP Lounge
Other CCPP (ccpp)

Informal Astro Talk
Matteo Biagetti
University of Geneva